PROJECT SUMMARY Older adults are the fastest growing segment of the U.S. population, with hearing loss the third most prevalent chronic medical condition. Untreated, hearing loss can interfere with effective communication, quality of life, and potentially accelerate cognitive decline. Cochlear implants (CI) have seen increasing use for older adults, with one implant center reporting a nearly 500% increase over a recent ten-year period in the number of individuals aged 60 or older receiving CIs. The auditory input provided by a CI is sharply degraded with respect to that of a person with normal hearing or even with mild to moderate hearing loss, but patients can adapt well to this novel input- at least when measured by standard clinical tests of word identification (e.g., CNC words) or perception of simple sentences (e.g., AzBio sentences). However, clinicians are often puzzled by patients, especially older adults, who seem to do well on standard tests but report major difficulties in everyday speech interactions. This grant builds upon the hypothesis that the combined constraints of the sharply degraded signal provided by a CI and finite cognitive resources may lead to a ?tipping point? when listeners are confronted with complex sentences and discourse arriving at normally rapid speech rates. Beyond this tipping point communication may become too difficult, even for CI recipients whose standard speech intelligibility scores are above average. This grant brings together a unique combination of three converging approaches: (1) We examine the ability of younger adult and older adult CI users to comprehend and remember the content of linguistically complex sentences and multi-sentence discourse, and probe potential effects on the cognitive mechanisms listeners use to process speech input, an underlying issue as yet unexplored in CI research; (2) For each experiment we will conduct a parallel study with normal-hearing younger and older adults using channel vocoded and unprocessed speech, with the former allowing a systematic determination of the interaction between signal degradation and the complexity of the speech materials on their recall, and the latter to illustrate a baseline of optimal performance on the same tasks for age-matched adults; (3) We will use modern pupillometry measures as an objective index of cognitive effort time-locked to the content of the speech input. The knowledge gained will test current models of resource allocation and listening effort as mechanisms underlying effects of cognitive load when younger and older adult CI users face the natural complexity of everyday sentences and discourse. Results of the studies will be valuable for counseling CI recipients and their families and will help develop more comprehensive evaluation batteries and rehabilitative strategies for the older adult CI recipient.